IMO, It does matter. The cast iron takes longer to heat up and it also takes longer to cool down, the steel or copper rads tend to heat up quicker, but on the down side, cool down quicker as well. If the thermostat is in an area which is being heated with cast iron rad, the rooms with the steel rads will cool down when thermostat isn't calling for heat.

However , if you can balance the house according to the type of rad , ei; a cast iron rad valve can be closed slightly to allow the steel rads to heat up for a longer period of time, then you will be good to go.

In one reply that is what I understand that if the thermo is in the cast room, the steel rad rooms will cool down to quickly.

Dual zoning would not be practical.

If there was a thermo on the steel rads, if the boiler was not on due to the main thermo in the cast room, it wouldn't make a difference cause heated water would not be "on" ? i.e. if the steel thermo "said" give me heat, there wouldn't be any to give cause the boiler would not be on, circulatory pump not running ?

Thoughts?

And P.S. If anyone knows were to purchase used cast rads w/o a second mortg. much appreciated.

You could let the circulator run, and let each room adjust it's needs based on its thermostat. These are totally mechanical, no electrical parts - they adjust the flow through the radiator to maintain the user set temperature. This also makes it easier to cool off a room that isn't being used to a predictable temperature. The boiler could be set to turn on when the return was a certain temp using aquastats, and abandon the central thermostat.

I really don't think a forced air system would work.
The house was built in 1930's and all the outside walls are brick and masonary. No insulation. No way to insulate unless the wall were built out - inside.

I believe a hydronic system the only way economically to supply a 'steady' heat. Forced air would be instant warm. instant cold ? I think.

If there was a thermo on the steel rads, if the boiler was not on due to the main thermo in the cast room, it wouldn't make a difference cause heated water would not be "on" ? i.e. if the steel thermo "said" give me heat, there wouldn't be any to give cause the boiler would not be on, circulatory pump not running ?

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If you replace the rad in the thermostat room, then the rest of the house would use the heat in that room to turn the boiler on and off,
If you make the rad vlave in that room work, you can adjust the amount of hot water running through the cast iron rad, allowing the steel rads to heat up longer. The rest of the house's heat would be gorverned by the room temperature of the thermostat location.

I find that you can usually pick up cast iron rads at a salvage yard (scrap metal dump) at a fraction of the price of new ones. Of course you will have to pressure test them . Some places will sell them by the pound . Scrap dealers usually offer about 3 cents per pound and you may end up picking one up for 5-20cents per pound from them (depending how desperate you look)

You can probably get better efficiency with a new forced-air furnace than a boiler would provide. There are multi-stage furnaces that will run for longer intervals, as they adjust the BTU input according to heating requirements. I have a project where steam heat may be replaced with forced air, for lower operating costs, not to mention central air.

It depends on the price point you are trying to end up with, but a two stage furnace with a variable speed fan would allow the system to run at low temp for a long time, keeping the temp even. Another thought is to use a high velocity system - the vents are very small and are designed to go in the ceiling. You'd often end up with more than one in a room depending on the size, which helps to keep temps more even.

You can probably get better efficiency with a new forced-air furnace than a boiler would provide. There are multi-stage furnaces that will run for longer intervals, as they adjust the BTU input according to heating requirements. I have a project where steam heat may be replaced with forced air, for lower operating costs, not to mention central air.

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Unless I am missing something here he has a hot water system. its possible to get a hot water condsing unit thats 98% efficient. They will also modulate and provide lower btu's when needed. They are most efficient when running at low temperature for longer periods of time. It will also provide a very comfortable heat.

Mowin - where are you located? In NYC, there's plumbing shops that re-fit & re-sell cast iron rads, pretty cheap. Last time I checked, a few years back, they were going about 25-50$.

That said, I'm not understanding why you can't mix cast & steel rads on the same system. The cast rads have a built-in thermal reservoir, so to speak, so there's a lag... but if the room with the thermostat has a steel rad - what's the problem?

As long as your replacements are sized properly, there's no reason cast and steel should/would transmit a different amount of heat per cycle. BTU's are BTU's. Am I missing something here?

I never suggested it wouldn't work. All I suggested was that the house needs to be rebalanced with the mixture of heating material elements. There are extra steps that need to be taken for the system to be balanced properly ei; the amount of heat producing water running through a particular materialed rad

Unless I am missing something here he has a hot water system. its possible to get a hot water condsing unit thats 98% efficient. They will also modulate and provide lower btu's when needed. They are most efficient when running at low temperature for longer periods of time. It will also provide a very comfortable heat.

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Must have been the hissing of my radiators making it hard to think. I still think 'steam' when I see the word radiator, since just about nobody wants cast-iron radiators in their homes, if they have a choice. (At least I didn't include a link to the Coal Burner website)

You can probably get better efficiency with a new forced-air furnace than a boiler would provide. There are multi-stage furnaces that will run for longer intervals, as they adjust the BTU input according to heating requirements. I have a project where steam heat may be replaced with forced air, for lower operating costs, not to mention central air.

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Wow.... For total system efficiency it is virtually impossible to beat hyrdonic heat. It is simply much more efficient to heat water and move it around than it is to create and move hot air.

That said, I'm not understanding why you can't mix cast & steel rads on the same system. The cast rads have a built-in thermal reservoir, so to speak, so there's a lag... but if the room with the thermostat has a steel rad - what's the problem?

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The only issue I can think of is if the radiators have significantly different thermal properties. Some steel rads have very low thermal mass. They are not designed to 'store' heat. Most cast iron radiators that I have seen have large thermal masses.